Page:Astronomy for Everybody.djvu/276

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PLANETS AND THEIR SATELLITES

The effects of this law require mathematical processes which it has taken two hundred years to bring to their present state, and which are still far from perfect. The measurement of the distance of a satellite is not a job to be done in an evening; it requires patient labor extending through months and years, and then is not as exact as the astronomer would wish. He does the best he can and must be satisfied with the result until he can devise an improvement on his work, which he is always trying to do with varying success.

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